1. Field of the Invention
The present invention relates to a light emitting device having a vertical structure, a package thereof and a method for manufacturing the same, and more particularly, to a light emitting device having a vertical structure, a package thereof and a method for manufacturing the same which are capable of damping impact generated in a substrate separation process, and achieving an improvement in mass productivity.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are well known as a semiconductor light emitting device which converts current to light, to emit light. Since a red LED using GaAsP compound semiconductor was commercially available in 1962, it has been used, together with a GaP:N-based green LED, as a light source in electronic apparatuses, for image display.
The wavelength of light emitted from such an LED depends on the semiconductor material used to fabricate the LED. This is because the wavelength of the emitted light depends on the band gap of the semiconductor material representing energy difference between valence-band electrons and conduction-band electrons.
Gallium nitride (GaN) compound semiconductor has been highlighted. One of the reasons why GaN compound semiconductor has been highlighted is that it is possible to fabricate a semiconductor layer capable of emitting green, blue, or white light, using GaN in combination with other elements, for example, indium (In), aluminum (Al), etc.
Thus, it is possible to adjust the wavelength of light to be emitted, using GaN in combination with other appropriate elements. Accordingly, where GaN is used, it is possible to appropriately determine the materials of a desired LED in accordance with the characteristics of the apparatus to which the LED is applied. For example, it is possible to fabricate a blue LED useful for optical recording or a white LED to replace a glow lamp.
On the other hand, initially-developed green LEDs were fabricated using GaP. Since GaP is an indirect transition material causing a degradation in efficiency, the green LEDs fabricated using this material cannot practically produce light of pure green. By virtue of the recent success of growth of an InGaN thin film, however, it has been possible to fabricate a high-luminescent green LED.
By virtue of the above-mentioned advantages and other advantages of GaN-based LEDs, the GaN-based LED market has rapidly grown. Also, techniques associated with GaN-based electro-optic devices have rapidly developed since the GaN-based LEDs became commercially available in 1994.
GaN-based LEDs have been developed to exhibit light emission efficiency superior over that of glow lamps. Currently, the efficiency of GaN-based LEDs is substantially equal to that of fluorescent lamps. Thus, it is expected that the GaN-based LED market will grow significantly.
Despite the rapid advancement in technologies of GaN-based semiconductor devices, the fabrication of GaN-based devices suffers from a great disadvantage of high-production costs. This disadvantage is closely related to difficulties associated with growing of a GaN thin film (epitaxial layer) and subsequent cutting of finished GaN-based devices.
Such a GaN-based device is generally fabricated on a sapphire (Al2O3) substrate. This is because a sapphire wafer is commercially available in a size suited for the mass production of GaN-based devices, supports GaN epitaxial growth with a relatively high quality, and exhibits a high processability in a wide range of temperatures.
Further, sapphire is chemically and thermally stable, and has a high-melting point enabling implementation of a high-temperature manufacturing process. Also, sapphire has a high bonding energy (122.4 Kcal/mole) and a high dielectric constant. In terms of a chemical structure, the sapphire is a crystalline aluminum oxide (Al2O3).
Meanwhile, since sapphire is an insulating material, available LED devices manufactured using a sapphire substrate (or other insulating substrates) are practically limited to a lateral or vertical structure.
In the lateral structure, all metal contacts for use in injection of electric current into LEDs are positioned on the top surface of the device structure (or on the same substrate surface). On the other hand, in the vertical structure, one metal contact is positioned on the top surface, and the other contact is positioned on the bottom surface of the device structure after removal of the sapphire (insulating) substrate.
In addition, a flip chip bonding method has also been widely employed. In accordance with the flip chip bonding method, an LED chip, which has been separately prepared, is attached to a sub-mount of, for example, a silicon wafer or ceramic substrate having an excellent thermal conductivity, under the condition in which the LED chip is inverted.
However, the lateral structure or the flip chip method suffers from the problems associated with poor heat release efficiency because the sapphire substrate has a heat conductivity of about 27 W/mK, thus leading to a very high heat resistance. Furthermore, the flip chip method has also disadvantages of requiring large numbers of photolithography process steps, thus resulting in complicated manufacturing processes.
To this end, LED devices having a vertical structure have been highlighted in that the vertical structure involves removal of the sapphire substrate.
In the fabrication of such a vertical LED structure, a laser lift off (LLO) method is used to remove the sapphire substrate, and thus, to solve the problems caused by the sapphire substrate.
However, it is impossible to completely remove the sapphire substrate at once, using the LLO method, due to the size and limited uniformity of a laser beam used in the LLO method. For this reason, uniform small-size laser beams are irradiated to respective portions of the sapphire substrate, in order to the entire portion of the sapphire substrate.
In the LLO method, stress is applied to the GaN thin film upon incidence of a laser beam. In order to separate a sapphire substrate and a GaN thin film from each other, it is necessary to use a laser beam having a high energy density. The laser beam resolves GaN into a metal element, namely, Ga, and nitrogen gas (N2).
The resolved nitrogen gas exhibits a high expansion force, so that it applies considerable impact not only to the GaN thin film 2, but also to a support layer for the GaN thin film 2 and metal layers required for the fabrication of the device. As a result, a degradation in bondability occurs primarily. In addition, a degradation in electrical characteristics occurs.
For example, wave patterns exhibited as having irregularities may be formed at the peripheral portion of the GaN thin film after completion of the LLO process. Also, during the LLO process, many poor bonding portions may be observed on the thin film.
Thus, the nitrogen gas generated during the LLO process damages the semiconductor layer arranged in the vicinity of he nitrogen gas. There may also be a phenomenon that cracks formed at poor-quality portions of the GaN thin film are propagated to other portions of the GaN thin film.
As apparent from the above description, a prolonged process is required in fabricating a desired device using a GaN thin film to form an LED layer. Furthermore, there are many difficulties in implementing this process. In particular, where separation of a substrate is carried out using a laser, nitrogen gas generated due to the laser may easily damage the thin films of a semiconductor layer arranged in the vicinity of the nitrogen gas. As a result, a degradation in productivity may occur.